The present invention relates to a process for the production of nicotinic acid by oxidizing di-alkylpyridine, i.e., 2-methyl-5-ethyl pyridine, 2,5-lutidine, 2-methyl-5-butyl pyridine and 2-propyl-5-ethyl pyridine, and more particularly to a process of producing nicotinic acid continuously with a nitric acid oxidation procedure without intricate post-treatment.
Nicotinic acid is a very important industrial product as a medication and an additive of foodstuff or feedstuff. Generally, it is made by oxidizing dialkylpyridine, paticularly 2-methyl-5-ethylpyridine (hereinafter 2-methyl-5-ethyl pyridine is abbreviated as MEP), and then decarboxylating the resulting oxidized compound and finally obtaining nicotinic acid. Further, as its oxidizing agent, air, sulphuric acid-nitric acid or nitric acid or the like is employed.
Except for the process using nitric acid as an oxidizing agent, the above mentioned processes have defects using as the formation of numerous secondary reaction products, impurities or colored finished products, and an inferior yield, and those processes are not practically employed as an industrial producing process.
Also, even the process that is an oxidizing procedure with nitric acid, and is the unique industrial process at present, gives an inferior yield or requires intricate operational steps or produces a poorly colored product, and is, therefore, still unsatisfactory as an industrial process.
The various processes heretofore known which comprise oxidizing MEP with nitric acid are summarized and classified roughly into the following three processes:
A. A process for producing nicotinic acid with two steps which comprise producing isocinchromeronic acid containing nicotinic acid, (the said isocinchomeronic acid is hereinafter abbreviated as (ICMA), in the first step and then applying a decarboxylating step to said isocinchomeronic acid after separating ICMA or with separating ICMA and thus obtaining nicotinic acid in the second step.
The reaction is carried out by oxidizing MEP with nitric acid by an amount of 90 to 117 percent to the theoretical oxidizing amount at a temperature of 175.degree. C to 200.degree. C under a gauge pressure of 18 to 35 kg/cm.sup.2 and thereby a resulting reaction solution containing mainly ICMA is obtained; and further, according to the process described in U.S. Pat. No. 2,702,802 and Zh. Prikl Khim 39(6)1388-94 (1966), U.S. S. R., the said ICMA is separated from the resulting reaction solution and a decarboxylating reaction is carried out in another reactor; and further, according to the process of French Patent Nos. 1,509,049 and 1,509,120, ICMA is not separated from the resulting reaction solution and the decarboxylating reaction is carried out at a temperature of 205.degree. C to 235.degree. C under pressure of 20 to 35 kg/cm.sup.2, and thus a solution containing nicotinic acid is obtained.
The separation of nicotinic acid from the reaction solution is carried out by neutralizing the reaction solution with a basic material to a pH value of 3.3 to 3.5 and cooling the reaction solution and crystallizing nicotinic acid.
According to these processes, nicotinic acid is crystallized as a product containing impurities and consequently, the product must be further purified and the operation for collecting unreacted MEP and nicotinic acid from a separated mother liquid is complicated. Furthermore, the reaction operation for the above process is carried out by two steps and additionally, an inferior yield of 70 percent or less acts as a drawback. The finished product obtained above is remarkably colored with a yellowish color and cannot be immediately used as a satisfactory finished product.
b. Another process, a process for continuously producing nicotinic acid is described in U.S. Pat. No. 2,708,196, which comprises taking out continuously a reaction solution containing ICMA and nicotinic acid from a reaction mixture, and further separating ICMA from the reaction solution and then using it recurrently for an initial reaction. Nicotinic acid is crystallized as a nitrate salt of nicotinic acid from a mother liquor, from which ICMA was separated. (Hereinafter, nitrate salt of nicotinic acid is abbreviated as NA-HNO.sub.3).
In the process, MEP is oxidized with nitric acid by an amount of 115 to 125 percent in proportion to the theoretical oxidizing amount of MEP at a temperature of 180.degree. C to 185.degree. C under a gauge pressure of 18 to 20 kg/cm.sup.2 and a reaction solution containing ICMA and NA-HNO.sub.3 is obtained, and then the reaction solution is continuously taken out and cooled at 35.degree. C for a sufficient length of time to separate ICMA. The ICMA is recurrently fed into a repeating step of the oxidizing reaction. On the other hand, a mother liquor containing NA-HNO.sub.3 and free of ICMA is cooled at a temperature ranging from 0.degree. C to -5.degree. C to separate NA-NHO.sub.3 from the mother liquor. But, the mother liquor, from which NA-HNO.sub.3 is once separated, still contains a large amount of NA-HNO.sub.3, so that the mother liquor is further concentrated and thereby, NA-HNO.sub.3 is collected from it. The NA-HNO.sub.3 obtained is treated by adding a basic material in it and a pH value of the resulting solution is adjusted in a range around an isoelectric point of nicotinic acid and then nicotinic acid is crystallized and separated by cooling it.
This process requires a long time for the step of separating ICMA, the step of separating NA-HNO.sub.3 and further the step of separating, crystallizing nicotinic acid, and furthermore this is a process requiring intricate operations, so that this process is unsatisfactory as an industrial process.
c. Another improved process which is adapted from the previous process (b) is provided by West German Patent No. 1,956,117 and Japanese Patent Publication No. 15944/1971.
This process is superior, because a high yield of 80 to 85 percent is attained and ICMA does not remain in a reaction terminated solution.
The process comprises carrying out the reaction by using nitric acid by an amount of 130 to 165 percent to the theoretical oxidizing amount of MEP at a temperature of 235.degree. C to 330.degree. C under a pressure of 50 to 280 kg/cm.sup.2.
This process does not necessitate the operation of separating ICMA but requires intricate operations for separating NA-HNO.sub.3 from the reaction solution and further separating, crystallizing nicotinic acid from it, such as revealed in the previous process (b). Therefore, this process is also unsatisfactory as an industrial producing process.
Another recent process is described by August Stocker et al in U.S. Pat. Nos. 3,657,259 and 3,741,976 directed to the production of the pyridine carboxylic acid hydronitrate. In this process it is necessary to separate the hydronitrate, dissolve it in ater, adjust the pH thereof to the isoelectric point of the desired acid product and recrystallize it therefrom.
The invention as well as other objects and advantages thereof will be more apparent from the following detailed description when considered with the accompanying drawings, in which: